1. Field of the Invention
The present invention relates to a heat-developable photosensitive material and an image forming method suitably usable in medical diagnosis, industrial photography, printing and computer output microfilming (COM).
2. Description of the Related Art
Various photosensitive materials comprising a substrate having disposed thereon a photosensitive layer on which images can be formed by imagewise exposure are known. Examples thereof include photosensitive materials that are thermally developed to form images, and are known as systems that contribute to environmental protection and can simply means for forming images.
Methods for forming images by thermal development are disclosed in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and in D. Klosterboer, “Thermally Processed Silver Systems”, page 279, chapter 9 in Imaging Processes and Materials: Neblette's, 8th Edition, edited by John Sturge, Vivian Walworth and Allan Shepp (London: John Wiley and Sons, Ltd., 1989). These heat-developable recording materials contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a silver reducing agent, all of which are dispersed in an organic binder matrix. While these heat-developable photosensitive materials are stable at room temperature, they produce silver when heated to a high temperature (e.g., 80° C. or higher) after exposure through a redox reaction between the reducible silver source (functions as an oxidizing agent) and the reducing agent. This redox reaction is promoted by catalytic action of a latent image generated by the exposure. The silver produced by the reaction of the reducible silver salt in the exposed region turns black and contrasts with the non-exposed region, whereby an image is formed.
As the heat-developable photosensitive material using an organic silver salt, a wide variety of reducing agents are disclosed, for example, in Japanese Patent Application Laid-Open (JP-A) Nos. 46-6074, 47-1238, 47-33621, 49-46427, 49-115540, 50-14334, 50-36110, 50-147711, 51-32632, 51-1023721, 51-32324, 51-51933, 52-84727, 55-108654, 56-146133, 57-82828, 57-82829, 6-3793, U.S. Pat. Nos. 3,667,9586, 3,679,426, 3,751,252, 3,751,255, 3,761,270, 3,782,949, 3,839,048, 3,928,686 and 5,464,738, German Patent No. 2,321,328, and European Patent No. 692,732.
Among these known reducing agents, a hindered phenol compound and a bisphenol compound have widely been used. However, heat-developable photosensitive materials using these reducing agents require time for development in order to obtain sufficient image density, and also there sometimes arises the problem of large variances in sensitivity with respect to development temperature. Techniques for solving this problem have been studied.
Development accelerators, and in particular reducing compounds, have been used to overcome this problem. For example, JP-A No. 10-221806 discloses a sulfonamide phenol compound. Additionally, U.S. Pat. No. 5,496,695 and JP-A No. 9-304875 disclose that ultrahigh-contrast performance can be obtained by using a hydrazine derivative as a known reducing compound in heat-developable photosensitive materials.
However, when the compounds disclosed in these publications are added to heat-developable photosensitive materials used for medical images and the like, where ultrahigh-contrast photographic characteristics are unnecessary, there arise problems such as severe fogging, excessive high contrast and poor image reproducibility, whereby it is impossible to obtain heat-developable photosensitive materials that are marketable as products.
When known development accelerators or known hydrazine derivatives such as the above are used, there are sometimes problems in that the effect of development acceleration is insufficient, contrast gradation is too high, image reproducibility is lowered, and storage stability of the photosensitive material is insufficient. These problems are caused by various factors such as combinations with other additives, manufacturing conditions of photosensitive materials, development temperature, and the passage of time, which constitute a large issue in designing heat-developable photosensitive materials. There has thus been a demand for a novel heat-developable photosensitive material that can solve these problems.